Frequency domain processing of scanning acoustic imaging signals

ABSTRACT

Method and apparatus useful in the inspection of a target comprises scanning the target with a pulsed acoustic beam, sensing the pulsed beam after it has been modified by interaction with the target, producing a time-domain signal indicative of the modifications, processing the time-domain signal to produce a frequency domain representation of the modifications, and producing an image-wise display of the frequency domain representation of the modifications. In one execution disclosed, the frequency domain representation is altered and then reconverted to a time domain signal before display.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a utility application based upon, and derivingpriority from, Provisional Application Ser. No. 60/248,138, filed Nov.13, 2000 which is owned by the owner of the present application.

BACKGROUND OF THE INVENTION

In a standard scanning acoustic microscope, a target is scanned with ahigh energy multi-megahertz acoustic beam pulsed at kilohertz rates. Thebeam as it passes through or is reflected from the target is modified inamplitude and/or phase.

The target may be inspected at various internal interfaces for defectsby collecting, amplifying and appropriately time-gating a reflectedfraction of the input signal. A greater gate delay represents a deeperreflected level in the target.

The most typical displays produced using this gated return signal willshow greater amplitude signals where the acoustic probe at the gateddepth is more strongly reflected than at other levels. By way ofexample, a strong reflection will occur if a disbond between two layersof an IC package has created an air gap, air being highly reflective ofacoustic waves traveling through a semiconductor medium.

Scanning acoustic microscopes utilizing such information displays haveproven to be of great benefit in nondestructive inspection and testingof semiconductor packages and many other commercial articles andlaboratory targets. To generate as much information as possible from thesensed acoustic beam, many image enhancements techniques have beendeveloped—colorization of differentiated information, edge enhancement,and so forth. Yet the desire for more and different information aboutinternal details in inspected targets continues to be intense andunabated.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for extracting valuable information from the output of ascanning acoustic microscope which is not generated using knowntechniques.

It is another object to provide such method and apparatus which is ofhigh resolution and accuracy.

It is still another object of the invention to provide such method andapparatus which is relatively inexpensive, and can be employed in realtime or with stored information.

It is yet another object to provide such method and apparatus whichcorrelates spatially with standard scanning acoustic microscope imageryand can therefore be employed as an image enhancing technique.

BRIEF DESCRIPTION OF THE FIGURES:

FIGS. 1-3 are schematic diagrams depicting three improved methods ofprocessing acoustic microscope signals according to the teachings of thepresent invention.

DESCRIPTION OF THE PREFERRED EXECUTIONS:

FIG. 1 is a schematic diagram illustrating a preferred execution of amethod aspect of the invention adapted for processing signals derivedfrom a scanning acoustic microscope pulser 10.

As is well known in the art a scanning acoustic microscope typicallycomprises a transducer which is driven by sharp voltage pulses which mayhave amplitudes of 100 volts or more and are typically in the frequencyrange of tens of megahertz to 100 megahertz or higher.

The pulsed acoustic beam penetrates the target, which may be an ICpackage, for example. A fraction of the energy passes through thetarget, and the remainder is absorbed or scattered. In many applicationssufficient energy is returned to the transducer (after a delay) to besensed. Acoustic energy is almost totally reflected by an air gap. Thusacoustic microscopes have proven to be extremely useful in locatingdisbonds (air gaps) between internal layers of a device such as an ICpackage.

The return signal is an amplitude signal composed of a range offrequencies centered around the transducer's resonant frequency. FIG. 1shows a receiver 12 adapted to sense and amplify the acoustic signalreturned from the target. The time domain signal after processing by thereceiver 12 has a waveform which resembles that sketched at 14. The timedomain signal 14 is representative of amplitude variations in thereturned acoustic pulses at the pixel level.

As is well known in the art, the time domain signal 14 is conventionallygated by a gating process shown schematically at 16. During the gatingprocess, a gate 18 isolates a pixel-representative signal segmentassociated with a single pixel. The gated waveform showing only thegated segment of the signal 14 is shown at 20.

Gating of the signal permits the user to examine any chosen level in thetarget simply by selecting an appropriate delay time for the gate. Forexample, a single pixel segment might be captured with a gate 100nanoseconds wide set at a delay of 384-484 nanoseconds. If a deeperlevel were to be visualized a longer delay would be employed.

The waveform shape of the signal segment 20 characterizes modificationsin the reflected amplitude of a particular acoustic pulse or pulsesimpinging on a pixel of the target. The modification may by caused byabsorption, scattering, reflection, interference or other effects andits capture in the signal segment 20 is highly useful to thoseinterested in a target's internal construction, defects and the like.

In accordance with standard practice in scanning acoustic microscopy,the gated pixel-wise signal segment 20 is subjected to a peak detectionstep 22 and then is displayed as a time domain acoustic image (see step24 in FIG. 1). A standard time domain acoustic image is shown at 26. Inthe image at 26, the target is an IC package; the darkened area in theupper left corner indicates a disbond where the reflected acousticenergy is significantly higher than in the remaining areas of thetarget.

In a broad sense the present invention is directed to a method ofprocessing a time-domain signal derived from an acoustic microscope,comprising converting the signal to a frequency domain representation ofthe signal. More particularly, with reference to FIG. 1, the gatedoutput time domain signal segment 20 is subjected to a frequency domainconversion step, preferably a Fourier transform, fast Fourier transform,discrete Fourier transform or other such well known signal processingsystems with windowing functions (see step 28 in FIG. 1).

Two outputs may be developed by the Fourier transform step—an amplitudeversus frequency waveform, sketched at 30, and a phase versus frequencywaveform, sketched at 32.

In accordance with an aspect of the present method, an output from theFourier transform step 28 is visually reproduced, as shown at 34. Theinformation content of the frequency domain characterization of thepixels (one of which is under discussion here) is in many casesdramatically different from that produced by a time domainvisualization. This can be noted even in the poorly reproduced picturesshown at 26 (time domain) and 34 (frequency domain). The pictures 26 and34 are taken from successful laboratory tests.

It must be understood that the particular waveforms 20, 30 and 32 areeach associated with a particular chosen pixel, whereas the time domainimage 26 and the frequency domain image 34 are images of the entiretarget or some macro portion thereof.

In accordance with the present invention, two methods are offered forselecting the frequency components of the signal which are to bevisualized in the frequency domain representation. FIG. 1 depicts one ofthe methods wherein in the frequency domain waveforms 30, 32, a singlefrequency (indicated at 36 on waveform 30) is selected. This may beaccomplished with Windows™ software which facilitates selection of theparticular chosen frequency under the control of a mouse.

The particular frequency 36 selected may, for example, be at the peak ofthe pixel-wise amplitude versus frequency waveform 30 as shown. Thatselected frequency then becomes the frequency component which isvisualized for all pixels in the display. Thus as the chosen frequency36 is varied along the frequency axis of signal segment 20, the visualappearance of the image 34 may change dramatically, indicating that theacoustic reflections from the target may vary widely at a particulartarget depth with the particular frequency being studied.

The frequency domain information alone is proving to be very valuable inproviding clues to hidden structures and anomalies within a target. Bysimultaneously displaying both time domain and frequency domain signalsside by side or superimposed, still further information can be derivedconcerning the target internal structures and anomalies. This subjectwill be discussed further in connection with the method of FIG. 2.

The particular site on the target where the determinate pixel ofinterest is located is preferably determined through Windows™ softwarewhich places a cursor under mouse or keyboard control at any desiredlocation on the target.

A second method of implementing the principles of the invention isdepicted schematically in FIG. 2. It is noted that the same referencenumerals appearing in different figures indicates like structure andfunction. Thus the FIG. 2 method may be the same as the FIG. 1 methoddescribed above except for the method step of selecting the frequencycomponent of the frequency domain waveform to be visualized.

Again, as in the FIG. 1 method, the output of the Fourier transform step28 may comprise an amplitude versus frequency waveform 44 and a phaseversus frequency waveform 46. However, rather than selecting a singlefrequency to be visualized for the chosen pixel and all pixels (that is,image-wise), a band 48 of frequencies is selected. The width andlocation of the band on the waveform 44 is preferably varied usingWindows™ software which permits under mouse or keyboard control, varyingof the location and width of the band 48 delimiters.

Thus the user may choose at will the width and location of the band offrequencies to be visualized. He might be interested in embracing arange of frequencies across the peak of the amplitude versus frequencywaveform. If the entire waveform is compressed at one of the spectrumfor example, he may wish to embrace the entire band of frequencies.Having selected the band and its location, in accordance with a step ofthe present invention (not shown), an algorithm is chosen which willproduce a single valued output in the application of the chosen band offrequencies to each pixel in the target to be processed and displayed.The algorithm may, for example, average the amplitudes of thefrequencies in the band, or choose the lowest value in the band 48 (seepoint 49 on waveform 44) or the highest amplitude value in the band (seepoint 51 on the waveform 44).

The waveform 44 is illustrated as having a curious dip within the chosenband 48 of frequencies for the selected pixel. This is the type ofinformation which likely would not be revealed in a rendition of aconventional peak-detected time domain signal. What might cause such adip? If the target included two closely spaced and parallel interfacesreflected acoustic waves could interfere constructively anddestructively. Interference would occur at certain frequencies and notat others. Thus the phenomenon is frequency selective.

With a broad band of reflected frequencies as normally occurs, theparticular band or bands of frequencies affected, and the distances inthe target corresponding to multiples of their wavelengths, couldsignify valuable interface spacing or other information. Thus the dip inthe band 48 could signify that interference is occurring in the span offrequencies across the dip.

If one pixel or pixel group has a dip as described and an adjacent pixelor pixel group does not, this fact may be shown in an image-wisefrequency domain display as an area of high contrast. The ability tovisualize or otherwise develop information about a target's internalstructure or anomalies which are undetectable using standard time domainimaging is one of advantages of Fourier transform signal processingaccording to the invention.

In FIG. 2, image 47 is a conventional time domain rendition of thetarget using conventional image processing as described. Image 45 is afrequency domain image produced using the Fourier domain conversiontechniques described, and using a band of frequencies rather that asingle frequency as in the FIG. 1 method. The visual differences in thetwo images are manifest, indicating the presence of new information inthe frequency domain image than is not present in the time domain image.

A third execution of the principles of the invention is depicted in FIG.3, where again the use of like reference numerals denotes like structureand function. In the FIG. 3 execution, rather than processing anddisplaying the frequency domain output from the Fourier transform step28 directly, its output is instead modified (step 52), as by any desiredshading, apodizing or other filter function, for example, and thenprocessed in an inverse Fourier transform step 54.

The output of the inverse Fourier transform step 54 is a gated timedomain signal 55 which will have the general appearance of a gated timedomain signal, but will differ from the gated time domain signal 20derived from the pulser 10, receiver 12 and gating 16 steps, as a resultof the predetermined filter function used to process the frequencydomain characterization of the pixel signal.

Thus each of the three executions of the invention described operate onthe frequency spectrum of an examined sample pixel—the first two methodsby the selection for display of the frequency component (singlefrequency or band of frequencies). The FIG. 3 method contemplates a moresophisticated or aggressive (than simply gating) phase and/or amplitudefiltering of the spectrum of frequencies in the return beam from theexamined sample location.

Whereas the preferred executions of the invention have been described ina method context, one skilled in the art will be cognizant of thesystems and software necessary to carry out the described methods andthis description is intended to embrace those structures.

Other alternatives and embodiments are contemplated. For example, theoutputs from the Fourier transform step 28 in the FIGS. 1 and 2executions could also be inverse transformed and displayed as timedomain signals. The pulser signal could be stored or processed in realtime. Executions of the principles of the invention other than thosedescribed are within the scope of the present invention and are intendedto be embraced by the following claims.

1. An acoustic micro imaging method useful in the inspection of atarget, comprising: scanning the target with a focused pulsed acousticbeam; sensing the pulsed beam after it has been modified by interactionwith the target and producing a time-domain signal indicative of themodifications; and processing said time-domain signal to produce afrequency domain representation of frequency selective modifications tothe pulsed acoustic beam produced by said interaction with said target,said frequency domain representation further comprising a plurality ofpixels where each pixel is amplitude modulated by a phase component ofthe sensed pulsed beam from a respective portion of the target.
 2. Themethod defined by claim 1 including inverse transforming said frequencydomain representation.
 3. The method defined by claim 2 includingmodifying said time-domain signal before processing it.
 4. The methoddefined by claim 2 including modifying said frequency domainrepresentation before said inverse transforming operation.
 5. The methoddefined by claim 4 wherein said modifying comprises apodizing, shading,or otherwise filtering said frequency domain representation.
 6. Themethod defined by claim 1 wherein said processing comprises Fouriertransforming.
 7. The method defined by claim 6 including inverse Fouriertransforming said frequency domain representation.
 8. An acoustic microimaging method useful in the inspection of a target, comprising:scanning the target with a focused pulsed acoustic beam; sensing thepulsed beam after it has been modified by interaction with the targetand producing a time-domain signal indicative of the modifications;processing said time-domain signal to produce a frequency domainrepresentation of frequency selective modifications to the pulsedacoustic beam produced by said interaction with said target; andproducing an image-wise display of said frequency domain representationof the modifications, said image-wise display further comprising aplurality of pixels where each pixel is amplitude modulated by a phasecomponent of the sensed pulsed beam from a respective portion of thetarget.
 9. The method defined by claim 8 including simultaneouslyproducing an image-wise display of said time-domain signal.
 10. Themethod defined by claim 9 including inverse transforming said frequencydomain representation to a time-domain signal, and wherein saidimage-wise display of said time-domain signal displays the result ofsaid inverse transforming.
 11. An acoustic micro imaging method usefulin the inspection of a target, comprising: scanning the target with afocused pulsed acoustic beam; sensing the pulsed beam after it has beenmodified by interaction with respective portions of the target andproducing a time-domain signal indicative of the modifications; gatingthe signal segment to isolate a pixel-representative segment thereof;converting the gate-isolated signal segment to a frequency domainrepresentation of frequency selective modifications to the pulsedacoustic beam produced by said interaction with said target, in thefrequency domain of the signal; identifying a selected frequencycomponent of the signal; and producing an image-wise signal representingmany pixels in an image wherein the information in thepixel-representative signal segment associated with each of said manypixels is amplitude modulated by a phase value of the said selectedfrequency component modified by the respective portion of the target.12. The method defined by claim 11 including inverse transforming saidfrequency domain representation.
 13. The method defined by claim 11including producing an image-wise display of said frequency domainrepresentation.
 14. The method defined by claim 11 includingsimultaneously producing an image-wise display of said time-domainsignal.
 15. The method defined by claim 14 including inverse Fouriertransforming said frequency domain representation to a time-domainsignal, and wherein said image-wise display of said time-domain signaldisplays the result of said inverse transforming.
 16. The method definedby claim 12 including modifying said frequency domain representationbefore said inverse transforming operation.
 17. The method defined byclaim 16 wherein said modifying comprises apodizing, shading, orotherwise filtering said frequency domain representation.
 18. The methoddefined by claim 11 wherein said converting comprises Fouriertransforming.
 19. A method of processing a time-domain signal derivedfrom an acoustic microscope, comprising converting the signal into afrequency domain representation of the signal that shows frequencyselective modifications to the signal caused by interaction of anacoustic signal with a target, said frequency domain representationfurther comprising a plurality of pixels where each pixel is amplitudemodulated by a phase component of the sensed pulsed beam from arespective portion of the target.
 20. The method defined by claim 19including producing an image-wise display of said frequency domainrepresentation.
 21. The method-defined by claim 20 includingsimultaneously producing an image-wise display of said time-domainsignal.
 22. The method defined by claim 19 wherein said convertingcomprises Fourier transforming.
 23. A method of processing a gatedtime-domain signal derived from a pulsed beam acoustic microscope,comprising Fourier transforming the signal to a frequency domainrepresentation of the signal showing frequency selective modificationsto the signal caused by interaction of an acoustic signal with a target,said frequency domain representation further comprising a plurality ofpixels where each pixel is amplitude modulated by a phase component ofthe gated time-domain signal from a respective portion of the target.24. A method of processing a time-domain signal derived from a scanningacoustic microscope, comprising: gating the signal to isolate apixel-representative segment thereof; converting the signal into afrequency domain representation of the signal that shows frequencyselective modifications to the signal caused by interaction of anacoustic signal with respective portions of a target; in the frequencydomain of the signal, identifying a selected frequency component of thesignal; and producing an image-wise signal representing many pixels inan image wherein the information associated with each of said manypixels is amplitude modulated by a phase value of the said selectedfrequency component of the pixel-representative signal segment from arespective portion of the target.
 25. The method defined by claim 24including inverse transforming said frequency domain representation. 26.The method defined by claim 24 including modifying said frequency domainrepresentation before said identification of a selected frequencycomponent of the signal.
 27. The method defined by claim 26 wherein saidmodifying comprising apodizing, shading, or otherwise filtering saidfrequency domain representation.
 28. The method defined by claim 24wherein said converting is a Fourier transform process.
 29. The methoddefined by claim 24 wherein said frequency component comprises a singlefrequency.
 30. The method defined by claim 29 including inversetransforming said frequency domain representation.
 31. The methoddefined by claim 24 wherein said frequency component comprises a band offrequencies.
 32. The method defined by claim 31 including inversetransforming said frequency domain representation.
 33. The methoddefined by claim 31 wherein said step of producing includes determiningthe dominant frequency in said selected band of frequencies.
 34. Amethod useful in the inspection of a target, comprising: scanning thetarget with a pulsed acoustic beam; sensing the pulsed beam after it hasbeen modified by interaction with the target and producing a firsttime-domain signal indicative of the modifications; processing saidtime-domain signal to produce a frequency domain representation offrequency selective, phase modifications to the pulsed acoustic beamproduced by said interaction with respective portions of said target;modifying said frequency domain signal using a predetermined filterfunction; and converting said modified frequency domain signal to asecond time-domain signal, where said second time-domain signal furthercomprises a plurality of frequency components from the respectiveportions of the target that have been modified by the predeterminedfilter function.
 35. The method defined by claim 34 including producingan image-wise display of said second time domain representation.
 36. Themethod defined by claim 35 including simultaneously producing animage-wise display of said frequency representation.
 37. The methoddefined by claim 34 wherein said converting comprises Fouriertransforming.
 38. The method defined by claim 34 including producing asimultaneous image-wise display of said first and second time-domainsignals.
 39. A method of processing a first time-domain signal derivedfrom an acoustic microscope, comprising converting the signal into afrequency domain representation of the signal that shows frequencyselective modifications to a phase of the signal caused by interactionof an acoustic signal with respective portions of a target, modifyingthe frequency domain representation of the signal using a predeterminedfilter function, and converting the modified signal to a secondtime-domain signal having a different image content than said firsttime-domain signal, where said second time-domain signal furthercomprises a plurality of phase versus frequency components fromrespective portions of the target that have been modified by thepredetermined filter function.
 40. The method defined by claim 39wherein said modifying comprises apodizing, shading, or otherwisefiltering said frequency domain representation.
 41. The method definedby claim 39 including producing an image-wise display of said secondtime domain representation.
 42. The method defined by claim 41 includingsimultaneously producing an image-wise display of said frequencyrepresentation.
 43. The method defined by claim 39 wherein saidconverting comprises Fourier transforming.
 44. The method defined byclaim 39 including producing a simultaneous image-wise display of saidfirst and second time-domain signals.
 45. A method of processing a gatedfirst time-domain signal derived from a pulsed beam acoustic microscope,comprising Fourier transforming the signal into a frequency domainrepresentation of the signal that shows frequency selectivemodifications to a phase of the signal caused by interaction of anacoustic signal with respective portions of a target, modifying thefrequency domain representation of the signal using a predeterminedfilter function, and inverse Fourier transforming the signal to a secondtime-domain signal having a different image content than said firsttime-domain signal, where said second time-domain signal furthercomprises a plurality of phase versus frequency components from therespective portions of the target that have been modified by thepredetermined filter function.
 46. A method of processing a firstimage-wise time-domain signal derived from an acoustic microscope,comprising: gating the signal to isolate a pixel-representative segmentthereof; converting the signal into a frequency domain representation ofthe signal that shows frequency selective modifications to a phase ofthe signal caused by interaction of an acoustic signal with respectiveportions of a target; in the frequency domain of the signal, identifyinga selected frequency component of the signal; modifying the frequencydomain representation of the phase of the signal using a predeterminedfilter function; and inverse Fourier transforming the signal to producea second image-wise time-domain signal representing many pixels in animage from respective portions of the target wherein the information inthe pixel-representative signal segment associated with at least anumber of said many pixels is amplitude modulated by respective phaseversus frequency components within the pixel-representative segment andwhere said amplitude modulation has been altered due to saidmodification of said signal by the predetermined filter function whilein the frequency domain.
 47. The method defined by claim 46 wherein saidmodifying comprising apodizing, shading, or otherwise filtering saidfrequency domain representation.
 48. The method defined by claim 46wherein said converting is a Fourier transform process.
 49. Apparatusfor inspecting a target and producing a frequency domain representationof a gated time domain signal, comprising: a scanning acousticmicroscope having an acoustic pulser that produces a focused, pulsedacoustic beam; a signal processor including an acoustic pulse sensor,amplifier and pixel gate coupled to said pulser that produces atime-domain representation of the acoustic beam after it has beenmodified by interacting with respective portions of the target; and afrequency domain converter responsive to the output of said signalprocessor that produces a frequency domain representation of frequencyselective modifications to a phase of the acoustic beam produced by saidinteracting with the target, said frequency domain representationfurther comprising a plurality of pixels where each pixel is amplitudemodulated by a phase component of the sensed pulsed beam from arespective portion of the target.
 50. The apparatus defined by claim 49including inverse transform circuitry coupled to said frequency domainconverter.
 51. The apparatus defined by 49 wherein said converter is aFourier transformer.
 52. Apparatus useful in the inspection of a target,comprising: means for scanning the target with a pulsed acoustic beam;means for sensing the pulsed beam after it has been modified byinteraction with the target and producing a time-domain signalindicative of the modifications; means for gating the signal to isolatea pixel-representative segment thereof; means for converting the signalinto a frequency domain representation of the signal that showsfrequency selective modifications to a phase of the signal caused byinteraction of an acoustic signal with respective portions of a target;means for identifying in the frequency domain a selected frequencycomponent of the signal; and means for producing an image-wise signalrepresenting many pixels in an image wherein the information in thepixel-representative signal segment associated with each of said manypixels is amplitude modulated by the phase of said selected frequencycomponent from a respective portion of the target.
 53. The apparatusdefined by claim 52 including means for producing an image-wise displayof said image-wise signal.
 54. The apparatus defined by claim 53including simultaneously producing an image-wise display of saidtime-domain signal.
 55. The apparatus defined by claim 52 wherein saidconverting comprises Fourier transforming.
 56. Apparatus useful in theinspection of a target, comprising: means for scanning the target with apulsed acoustic beam; means for sensing the pulsed beam after it hasbeen modified by interaction with the target and producing a firsttime-domain signal indicative of the modifications; means for processingsaid time-domain signal to produce a frequency domain representation offrequency selective modifications to a phase of the pulsed acoustic beamproduced by said interaction with said target where said frequencydomain representation is amplitude modulated by a phase of a frequencycomponent of the sensed pulsed beam from a respective portion of thetarget.; means for modifying said frequency domain signal using apredetermined filter function; and means for converting said modifiedfrequency domain signal to a second time-domain signal.
 57. Theapparatus defined by claim 56 including means for producing animage-wise display of said second time domain representation.
 58. Theapparatus defined by claim 57 including means for simultaneouslyproducing an image-wise display of said frequency representation. 59.The apparatus defined by claim 56 wherein said converting comprisesFourier transforming.
 60. The apparatus defined by claim 56 includingmeans for producing a simultaneous image-wise display of said first andsecond time-domain signals.
 61. Apparatus for processing a firsttime-domain signal derived from an acoustic microscope, comprising meansfor converting the signal into a frequency domain representation of thesignal that shows frequency selective modifications to a phase of thesignal caused by interaction of an acoustic signal with respectiveportions of a target, means for modifying the frequency domainrepresentation of the signal, and means for converting the modifiedsignal of respective portions of the target to a second time-domainsignal having a different image content than said first time-domainsignal where said second time-domain signal further comprises aplurality of phase components that have been modified by the means formodifying.
 62. The apparatus defined by claim 61 including means forproducing an image-wise display of said second time domainrepresentation.
 63. The apparatus defined by claim 62 including meansfor simultaneously producing an image-wise display of said frequencyrepresentation.
 64. The apparatus defined by claim 61 wherein said meansfor converting comprises Fourier transforming.
 65. The apparatus definedby claim 61 including means for producing a simultaneous image-wisedisplay of said first and second time-domain signals.
 66. The methoddefined by claim 1 including modifying said time-domain signal beforeprocessing it.
 67. The method defined by claim 66 wherein said modifyingcomprises windowing, shading, or otherwise filtering said time-domainsignal.
 68. The method defined by claim 31 wherein said step ofproducing includes determining the lowest amplitude in said selectedband of frequencies.
 69. The method defined by claim 38 includingproducing a third image-wise display of said frequency-domainrepresentation prior to converting.
 70. The method defined by claim 42including producing a third simultaneous image-wise display of saidfirst time-domain signal.
 71. The method defined by claim 44 includingproducing a third simultaneous image-wise display of saidfrequency-domain representation.
 72. The apparatus defined by claim 60including means for producing a third simultaneous image-wise display ofsaid frequency representation.
 73. The apparatus defined by claim 65including means for producing a third simultaneous image-wise display ofsaid frequency representation.
 74. The method defined by claim 1including digitizing said time-domain signal before processing it. 75.The method defined by claim 66 including inverse transforming saidfrequency-domain representation.
 76. The method defined by claim 11including digitizing the gate-isolated signal segment prior toconverting it.
 77. The method defined by claim 68 including inversetransforming said frequency-domain representation.